Refractometer and method for measuring refractive index

ABSTRACT

The invention relates to a refractometer and to a refractive index measuring method. The invention consists in directing an incident light ray onto an interface consisting of a medium of known refractive index and of the medium studied, then in measuring the intensity of the reflected light ray. The ratio between the intensity of the incident ray and the intensity of the reflected ray allows the refractive index of the medium studied to be calculated by means of Fresnel&#39;s formulas.

FIELD OF THE INVENTION

[0001] The present invention relates to the sphere of determination of the refractive index of a medium. It provides a method and a refractometer for absolute measurement of the refractive index.

[0002] Determination of the refractive index of a medium has many applications, notably recognition of a compound and/or of the composition of a mixture, quality control in industrial production.

[0003] In the present description, the refractive index of a medium is the ratio of the propagation velocity of light in a vacuum divided by the propagation velocity of light in this medium.

[0004] A light ray is an electromagnetic wave whose wavelength belongs to the ultraviolet, visible and infrared waves domains, and to the radio waves domain.

[0005] The intensity of a light ray is the flux of photons traversing a surface for a given time.

BACKGROUND OF THE INVETION

[0006] Current methods of determining the refractive index of a medium are based on Descartes' laws describing the refraction phenomenon upon passage of a light ray through an interface consisting of the medium to be studied and of a medium of known refractive index. These methods generally measure characteristics of the ray refracted by the interface to determine the refractive index of the medium studied. More particularly, some methods exploit the existence of a limit angle of refraction. These methods consist in determining the angle of an incident ray for which reflection is total and refraction non-existent. This angle corresponds to what is referred to as critical angle.

[0007] The refractometers measuring characteristics of a refracted ray or of the critical angle can be classified into two intrument categories.

[0008] On the one hand, there are refractometers performing absolute measurement of the refractive index. These tools allow, among other things, to control the quality of a product, to identify a component or to determine the proportions of the various components of a mixture. They are commonly used in the chemical, pharmaceutical and food-processing industry, and in analysis laboratories.

[0009] On the other hand, there are refractometers performing differential refractive index measurement between two media. The most widely known application of this category of instruments is analysis of the composition of a liquid solution after separation, by chromatography, of the different components upon passage of the liquid solution on an adsorbent solid.

[0010] Document EP-043,667 describes a refractometer using the comparison between the intensity of a first reflected ray and the intensity of a second reflected ray. The intensity of the first reflected ray depends on the refractive index of the medium studied, whereas the intensity of the second reflected ray is independent of the refractive index of the medium studied. However, the intensity value of the second ray is totally independent of the intensity of the first ray. Intensity measurement of two different light rays notably has the drawback of following a complex procedure for measuring a refractive index and of losing measuring precision.

[0011] Document U.S. Pat. No. 3,650,631 describes a refractometer using the comparison between a first reflected ray and the intensity of a second ray reflected on an interface consisting of the medium studied and of a reference solid. The first ray falls onto the interface with an angle of incidence which is smaller than the critical angle, and the second ray falls onto the interface with an angle of incidence larger than the critical angle. The refractometer described in document U.S. Pat. No. 3,650,631 requires a critical angle. Now, the critical angle is determined by the refractive indices of the media that constitute the interface. This requirement therefore imposes limitations on the refractive index value measurable by the refractometer of document U.S. Pat. No. 3,650,631. Furthermore, determination of the refractive index is based on the measurement of the intensity of the second ray which undergoes a total reflection on the interface. Now, in total reflection (angle of incidence larger than the critical angle), the evanescent wave appears, i.e. a phenomenon where the incident ray of the reference medium passes into the medium to be studied before it is reflected in the reference medium. In case of a highly absorbent studied medium, the intensity of the second reflected ray is attenuated and determination of the refractive index loses precision.

[0012] However, several limitations of use are set to refractometers measuring characteristics of the refracted light ray or the critical angle.

[0013] One limitation concerns the restricted measuring range of the refractive index of the medium studied. An absolute-measuring refractometer generally scans a range between approximately 1.3 and 1.7 RIU (Refractive Index Unit) for liquid media and between approximately 1.1 and 1.2 RIU for gaseous media, with a sensitivity of about 10⁻⁴ RIU. The measuring range of a differential-measuring refractometer extends approximately over 10⁻³ RIU with a sensitivity of 10⁻⁷ RIU.

[0014] Furthermore, the measurement based on the characteristic of the refracted ray prevents determination of the refractive index of an opaque medium because the refracted ray is absorbed by the opaque medium. By determining the angle from which reflection is total, the evanescent wave, a phenomenon which causes passage of the incident ray on an interface from the first medium into the second medium before it is reflected in the first medium, is also absorbed by the second medium if it is opaque. Thus, measurement of the refractive index of media such as, for example, crude petroleum, inks and paints is inaccessible with such instruments.

[0015] Besides, determination of the refractive index of a flowing fluid can be imprecise. In fact, if the fluid has no homogeneous optical characteristics as a result of its flow, the refracted ray or the evanescent wave propagated in the fluid undergoes alterations. The changes in the characteristics of the refracted ray or of the evanescent wave are all the greater as the flow of the fluid is turbulent.

[0016] For the same reasons as for determination of the refractive index of a flowing fluid, determination of the refractive index of a disperse medium can be imprecise because disperse media have no homogeneous optical characteristics.

[0017] The goal of the present invention is to provide a method and a refractometer allowing absolute measurement of a refractive index and overcoming the limitations of the prior art.

SUMMARY OF THE INVENTION

[0018] The invention consists in directing an incident light ray onto an interface consisting of a medium of known refractive index and of the medium studied, then in measuring the intensity of the reflected light ray. The ratio between the intensity of the incident ray and the intensity of the reflected ray allows the refractive index of the medium studied to be calculated by means of Fresnel's formulas.

[0019] One of the difficulties linked with measurement of the intensity of the reflected light ray lies in its low intensity.

[0020] The invention relates to a method of measuring the refractive index of a first medium, comprising the following stages:

[0021] directing an incident light ray onto a first interface consisting of the first medium and of a second medium of known refractive index so as to produce a reflected light ray,

[0022] measuring the intensity of said incident light ray and the intensity of said reflected light ray,

[0023] determining the refractive index of the first medium by taking account of at least the refractive index of the second medium, the intensity of the incident light ray and the intensity of the reflected light ray.

[0024] The refractive index of the first medium can be determined by means of a formula which relates the intensity of the incident light ray to the intensity of the reflected light ray at the first interface by taking account of the refractive index of the first medium and of the refractive index of the second medium, for example Fresnel's formulas.

[0025] In the method according to the invention, the incident light ray can be coded for example by varying the intensity of the incident light ray according to a periodic signal and/or by varying the mean intensity of the incident light ray according to a square-wave signal.

[0026] In the method according to the invention, a source light ray can be divided so as to form a reference light ray and said incident light ray, for example by means of a second interface.

[0027] The ratio of the intensity of said reference light ray to the intensity of said incident light ray can be determined using said first medium whose refractive index is known, by measuring the intensity of said reference light ray and the intensity of the reflected light ray and using a formula, for example Fresnel's formulas, which relates the intensity of the incident light ray to the intensity of the reflected light ray at said first interface by taking account of the refractive indices of the first medium and of the second medium.

[0028] In the measuring method according to the invention, the intensity of the reference light ray can be measured to determine the intensity of the incident light ray.

[0029] In the measuring method according to the invention, the intensity of the light ray refracted in the first medium is measured and the absorption in the first medium of the refracted light ray is determined.

[0030] In the measuring method according to the invention, the angle of incidence of the incident light ray in relation to the normal direction to the first interface can be smaller than the critical angle and preferably below 10°.

[0031] The method according to the invention can be applied to measurement of the refractive index of an opaque medium such as a crude petroleum for example.

[0032] The invention also relates to a refractometer comprising a light source, a first interface consisting of a first medium of unknown index and of a second medium of known index and at least one detector measuring the intensity of an incident light ray and the intensity of a reflected light ray resulting from the reflection of said incident light ray on said first interface. The first medium can be contained in an enclosure consisting of a shell and of the second medium.

[0033] The refractometer according to the invention can comprise a means for coding the light ray emitted by said light source and a means for decoding the measurements performed by said detector, said decoding means exchanging information with the coding means. The refractometer can also comprise an optical element, for example a second interface formed by the surface of the first medium located outside the enclosure or a semireflecting blade, suited to divide a light ray into two light rays so as to form a measuring ray and said incident ray, the intensity of said measuring ray being measured by a detector.

[0034] The refractometer according to the invention can comprise at least one detector measuring the intensity of a light ray refracted in the first medium.

[0035] According to the refractometer of the invention, the angle of incidence of the incident light ray in relation to the normal direction to the first interface can be smaller than the critical angle, preferably below 10°.

[0036] The method and the refractometer according to the invention notably have the advantage of providing a measuring range that can be included between 1 and 2.4 RIU for all media, in particular liquid and gaseous fluid media. Since the ray subjected to measurement does not pass through the medium studied, in comparison with the refracted ray, or does not enter the medium studied as much as the evanescent wave for an incident ray in the vicinity of the critical angle, it is possible to determine the refractive index of an opaque medium, of a flowing fluid medium or of a disperse medium.

[0037] Direct measurement of the intensity of the incident ray and of the reflected ray simplifies the refractive index measuring procedure and improves the measuring accuracy. According to the present invention, the absolute refractive index can be obtained with a precision at least equal to 10⁻⁴ RIU.

[0038] The refractometer according to the invention requires no prior calibration, whether optical, mechanical or electric, before measurement.

[0039] According to the layout of the refractometer of the invention, the medium to be studied is separated from the other means of the refractometer by an enclosure consisting of the medium of known refractive index forming part of the interface and possibly of a shell. It is thus possible to measure the refractive index of a medium under pressure. Furthermore, by selecting a suitable material and geometry for the medium of known index and for the shell, the refractometer has a high mechanical and/or chemical strength. Besides, separation of the medium to be studied from the other elements of the refractometer facilitates maintenance of the device.

[0040] Furthermore, the refractometer according to the invention can be easily completed with means allowing to measure the phenomenon of absorption of the medium studied.

BRIEF DESCRIPTION OF THE FIGURES

[0041] Other details, features and advantages of the invention will be clear from reading the description hereafter of an embodiment of the invention given by way of non limitative example, with reference to the accompanying figures wherein:

[0042]FIG. 1 shows the principle of a refractometer according to the invention,

[0043]FIG. 2 diagrammatically shows a refractometer according to the invention,

[0044]FIG. 3 shows an element of the refractometer,

[0045]FIG. 4 diagrammatically shows the phenomenon of refraction and reflection of a light ray directed onto an interface.

DETAILED DESCRIPTION

[0046] According to the refractometer diagrammatically shown in FIG. 1, medium 5 of unknown refractive index is contained in an enclosure delimited by shell 6. A window in shell 6 is closed by a medium 4 of known refractive index. An ambient medium 12 of known refractive index is located outside the enclosure.

[0047] The enclosure can have various forms, its main purpose being to contain medium 5. Thus, the enclosure can be a pipe in which medium 5 circulates, an open or closed vessel. Since medium 5 can be under pressure and/or an active chemical agent, the enclosure is designed to withstand the mechanical and/or chemical stresses applied thereto by medium 5. Owing to its shape and to its material, medium 4 must have the same mechanical and chemical strength characteristics as shell 6.

[0048] The surface of medium 4 in contact with medium 5 forms an interface 7. Medium 4 is selected in such a way that it absorbs no or only a very small proportion of light, its refractive index is known with a precision of at least 10⁻⁴ RIU and the surface forming interface 7 produces no unwanted interference, diffusion or diffraction effects. Medium 4 can for example have the shape of a diamond porthole of refractive index 2.41 RIU.

[0049] Ambient medium 12 must have no effect on the light. It can consist of a controlled atmosphere or, more simply, of ambient air.

[0050] The refractometer comprises a light source 1. Light source 1 can emit a monochromatic light ray by means of a filter, for example, whose stability is 10⁻⁴ for the mean intensity emitted. It can consist of a laser diode or a laser emitting for example a ray whose intensity is about 5 mW. Coding means 2 allow the light ray emitted by source 1 to be coded. Light source 1 and coding means 2 are arranged in ambient medium 12.

[0051] An optical element 3 arranged in ambient medium 12 between light source 1 and interface 7 allows to divide an incident light ray into two light rays whose characteristics are identical to those of the incident light ray, but of different intensities. The sum of the intensities of the two rays from optical element 3 can be equal to the intensity of the incident ray. Optical element 3 can be a semireflecting blade or an interface.

[0052] Two photodetectors 9 and 10 suited to measure the intensity of a light ray are arranged in ambient medium 12. Medium 5 comprises a photodetector 13 intended to measure the intensity of a light ray. Photodetector 13 is mobile in translation so as to vary the distance that separates it from interface 7. It is also possible to maintain photodetector 13 outside medium 5 and to use a glass finger allowing to drive a light ray from a first end of the finger, dipped in medium 5, to the second end, outside medium 5. Photodetector 13 measures the intensity of the light ray coming from said second end of the glass finger. The glass finger, secured to photodetector 13, can be mobile in translation. Furthermore, owing to the nature of its material, the glass finger does not modify the intensity value of the light ray it transmits.

[0053] Signal processing means 11 are connected to coding means 2 and to photodetectors 9, 10 and 13. They allow to decode the signals picked up by photodetectors 9, 10 and 13 considering the coding performed by means 2, and to compare and analyse the signals picked up by photodetectors 9, 10 and 13.

[0054] The refractometer can be provided with refrigeration means in order to eliminate the energy coming notably from light source 1 and thus to prevent modification of the value of the refractive index of medium 5 which varies by about 10⁻⁴ RIU per Celsius degree.

[0055] Light source 1, provided with coding means 2, allows to produce light ray A. Light ray A, after traversing ambient medium 12, enters optical element 3 where it is separated into two light rays B and C.

[0056] Photodetector 9 measures the intensity of this light ray C.

[0057] At the outlet of optical element 3, light ray B is propagated into ambient medium 12, then into medium 5 up to interface 7 to produce a reflected light ray E and a refracted light ray D.

[0058] The intensity ID of light ray D refracted in medium 5 is measured by photodetector 13 in order to determine the transmission of medium 5. The direction of translation of photodetector 13 is selected preferably parallel to the direction of the light ray. The photodetector performs at least one measurement of intensity ID at a distance d between photodetector 13 and interface 7. Photodetector 13 preferably performs at least three measurements of the intensity ID₁, ID₂ and ID₃ of ray D at respectively three distances d₁, d₂ and d₃ between photodetector 13 and interface 7. If photodetector 13 is provided with a glass finger, distances d, d₁, d₂ and d₃ correspond to the distance between interface 7 and the first end of the glass finger because the glass finger does not modify the intensity ID of ray D it picks up and transmits to photodetector 13.

[0059] Ray E is propagated in medium 4, then it leaves it and enters ambient medium 12. Photodetector 10 measures the intensity IE of this light ray E.

[0060]FIG. 2 shows another embodiment of the refractometer according to the invention. The reference numbers in FIG. 2 which are identical to those of FIG. 1 designate the same elements. Light source 1, coding means 2, medium 4, medium 5, shell 6, interface 7, signal processing means 11, medium 12, photodetectors 9, 10 and 13 can thus also be seen in FIG. 2, with the same layout as in FIG. 1.

[0061] In FIG. 2, optical element 3 has the form of an interface 8 formed by the interface between ambient medium 12 and medium 4. The two parts of medium 4 forming interfaces 7 and 8 consist of two planes. These two planes can be parallel as shown in FIG. 2, or form an angle a as shown in FIG. 3.

[0062] Photodetector 13 is provided with a glass finger 14 in form of a cylinder.

[0063] In FIG. 2, light source 1 provided with coding means 2 produces a light ray A. The angle of incidence of light ray A in relation to the normal direction N to interface 8 is α1. At interface 8, ray A is separated into a light ray C reflected in ambient medium 12 and a light ray B refracted in medium 4.

[0064] Reflected light ray C forms an angle α3 in relation to the normal direction N to interface 8. According to the reflection phenomenon, angle α3 is equal to angle α1. Photodetector 9 measures the intensity IC of light ray C.

[0065] Light ray B forms an angle α2 in relation to the normal direction N to interface 8. Interface 8 being a plane perpendicular to the plane forming interface 7, light ray B forms an angle α2 in relation to the normal direction N to interface 7. The value of angle α2 is imposed by Descartes' light ray refraction laws: n12.sin(α1)=n4.sin(α4) with n12 the refractive index of ambient medium 12 and n4 the refractive index of medium 4. At interface 7, light ray B is separated into a light ray E reflected in medium 4 and a light ray D refracted in medium 5.

[0066] Light ray D is propagated into medium 5, then it enters glass finger 14 through a first end. Photodetector 13 measures the intensity ID of light ray D which leaves through a second end of glass finger 14. Photodetector 13 performs at least three measurements of intensity ID₁, ID₂ and ID₃ of ray D at respectively three distances d₁, d₂ and d₃ between glass finger 14 and interface 7.

[0067] Light ray E forms an angle α4 in relation to the normal N to interface 8. According to the reflection phenomenon, angle α4 is equal to angle α2. Light ray E is propagated in medium 4, then it refracts at interface 8 in ambient medium 12 with an angle α5 in relation to the normal direction N to interface 8. Interface 8 being a plane perpendicular to the plane forming interface 7, light ray E forms an angle α4 in relation to the normal direction N to interface 8. The value of angle α5 is imposed by Descartes' light ray refraction laws: n4.sin(α4)=n12.sin(α5) with n12 the refractive index of ambient medium 12 and n4 the refractive index of medium 4. The intensity IE of light ray E is measured by photodetector 10.

[0068] According to the configuration of FIG. 2, angle α3 is equal to angle α5. By imposing the distances that separate the various elements of the refractometer, the location of these elements is defined with precision. When using a medium 4 according to FIG. 3, the angle of inclination of interface 7 in relation to interface 8 has to be taken into account to define the trajectory of the various light rays.

[0069] Photodetectors 9 and 10 can be replaced by a single photodetector measuring successively and alternately the intensity IC of light ray C and then intensity IE of light ray E.

[0070] The layout of the components of the refractometer according to the invention is so selected that the angle of incidence of light ray B in relation to the normal direction N to interface 7 is smaller than the critical angle, if there is one. In fact, according to Fresnel's formulas, the intensity of the reflected light ray E depends on the refractive index of medium 5 for any angle of incidence of light ray B between 0° and the critical angle of total reflection. For an angle of incidence of light ray B larger than the critical angle, the intensity of light ray B can be absorbed in medium 5 because of the evanescent wave phenomenon.

[0071] Preferably, the layout of the components of the refractometer according to the invention is so selected that the angle of incidence of light ray B in relation to the normal direction N to interface 7 is below 10°, advantageously below 3° and preferably zero. In fact, the intensity of reflected light ray E depends on the state of polarization of light ray B, except when the angle of incidence of light ray B is small (at least below 10°, advantageously below 3° and preferably zero). As a consequence, when using a small angle of incidence of light ray B, the intensity of light ray E is hardly influenced by the polarization of light ray B, therefore by the polarization of the ray emitted by light source 1. This allows to increase the precision and the reliability of the refractive index measurements according to the present invention.

[0072] The reflection phenomenon producing on interface 7 a reflected light ray E of an intensity of the order of some hundredths of the intensity of incident ray B, optical element 3 is so selected that the intensity IC of light ray C also represents some hundredths of the intensity IB of light ray B. Thus, signal processing means 11 analyse, by means of photodetectors 9 and 10, rays having comparable intensity values. This is preferable for the electronic amplification and comparison elements of signal processing means 11 and improves the precision of the measurements and of the results.

[0073] Coding means 2 allow the light ray produced by light source 1 to be provided with recognizable characteristics. The intensity of light ray A can vary periodically, for example by varying sinusoidally at fixed frequencies of about 10 kHz, around a given mean value. Since the sinusoidal variation of the intensity of light ray A is not affected by the effects of optical element 3, or by the reflection and refraction phenomena, the intensities of light rays C, D and E similarly vary sinusoidally at about 10 kHz, but around a different mean value. Signal processing means 11 are informed of the coding carried out by means 2 and can distinguish, from among the information provided by detectors 9, 10 and 13, the intensities of light rays C, D and E from the other parasitic signals such as those of the ambient light.

[0074] Furthermore, coding means 2 can produce a light ray of square-wave intensity with intermittences between a light ray of full intensity and a light ray of zero intensity. The square-wave signal can consist of a full-intensity phase during a time t₁ of 1 second, then of a zero-intensity phase during a time t₂, for example 2 seconds. The cycle consisting of the succession of times t₁ and t₂ is repeated throughout the measuring stage. The energy locally absorbed by medium 5 at interface 7 during the full-intensity phase has time to diffuse and dissipate in medium 5 during the zero-intensity phase. Thus, the variation of the refractive index value of medium 5 due to the temperature rise remains low. Coding of the intensity of light ray A in form of a square-wave signal is particularly suited for measurement of the refractive index of opaque media which absorb light quite rapidly.

[0075] Reflection of a light ray, a phenomenon on which the invention is based, is described in connection with FIG. 4 for example by Fresnel's formulas relating the intensity Ii of incident ray i to the intensity Ire of reflected light ray re at the level of an interface q as a function of refractive indices n1 and n2 of the two media 1 and 2 forming the interface. Ray ra represents the ray refracted in medium 2. Fresnel's formulas describe the phenomenon of reflection and polarization of the incident light.

[0076] Without departing from the scope of the invention, it is possible to use any formula equivalent to Fresnel's formulas describing the reflection phenomenon on an interface from the point of view of the intensities of the incident and reflected rays, by taking into account the refractive indices of the two media forming the interface. When the angle of the incident ray i on interface q is zero, Fresnel's formulas are written in form of a single expression: $\frac{I_{i}}{I_{r}} = \frac{\left\lbrack {\left( \frac{n_{2}}{n_{1}} \right) - 1} \right\rbrack^{2}}{\left\lbrack {\left( \frac{n_{2}}{n_{1}} \right) + 1} \right\rbrack^{2}}$

[0077] According to the invention, by measuring the intensity IC of ray C and knowing the distribution of the intensity IA of ray A between rays B and C by means of optical element 3, the intensity IB of ray B is determined. By measuring the intensity IC of ray C to know the intensity IB of ray B, a more accurate measurement is obtained because the value of intensity IC is comparable to intensity IE.

[0078] After determining the intensity IB of light ray B, by measuring the intensity IE of light ray E, and knowing the refractive index n4 of medium 4, the refractive index n5 of the medium studied 5 is determined by means of Fresnel's formulas.

[0079] The reflection and refraction phenomena, notably at the level of optical element 3, as ray B passes from medium 12 to medium 4 and ray E passes from medium 4 to medium 12, can be advantageously disregarded because their influence on intensities IB and IE are comparable.

[0080] In order to precisely determine, as ray A passes through optical element 3, the value of the ratio of intensity IC of light ray C divided by the intensity IB of light ray B, a medium 5 of known refractive index n5 is placed in the refractometer according to the invention. The intensity IB of light ray B is determined using Fresnel's formulas, knowing the refractive indices n4 and n5 of media 4 and 5 and by measuring the intensity IE of light ray E. Since the intensity IC of light ray C is measured, the ratio of intensity IC to intensity IB is known with precision, said ratio being used for implementing the refractometer according to the invention in order to determine the refractive index of a medium 5. This ratio is known as the “measuring device constant”.

[0081] The refractive index n of a medium can be written in form of a complex number n=r+i.χ, r being the real part and χ the imaginary part. Imaginary part χ describes the phenomenon of absorption of a ray propagating in an opaque medium. While traversing medium 5, the intensity ID of ray D decreases as a function of the distance d travelled from interface 7, according to a law of the form: ID(d)=k.e^(−χ.d), k being the value of the intensity ID of ray D at interface 7, i.e. for d=0.

[0082] Thus, by measuring the intensity ID of ray D at least at a distance d between photodetector 13 and interface 7, preferably by measuring at least three intensities ID₁, ID₂ and ID₃ at respectively three distances d₁, d₂ and d₃ between photodetector 13 and interface 7, the imaginary part χ5 of medium 5 can be determined. If photodetector 13 is provided with a glass finger, distances d, d₁, d₂ and d₃ correspond to the distance between interface 7 and one end-of the glass finger.

[0083] Measurement of imaginary part χ allows to characterize more precisely the medium studied and it notably allows to evaluate the absorbent character of the medium studied.

[0084] The refractometer according to the invention thus allows to determine simultaneously, at a given wavelength, the value of the refractive index and the absorption value of a medium studied. 

1) A method of measuring the refractive index of a first medium, comprising the following stages directing an incident light ray onto a first interface consisting of first medium and of a second medium of known refractive index so as to produce a reflected light ray, measuring the intensity of said incident light ray and the intensity of said reflected light ray, determining the refractive index of first medium by taking account of at least the refractive index of second medium, the intensity of the incident light ray and the intensity of the reflected light ray. 2) A method as claimed in claim 1, wherein the refractive index of first medium is determined by a formula relating the intensity of the incident light ray to the intensity of the reflected light ray at first interface by taking account of the refractive index of first medium and of the refractive index of second medium. 3) A method as claimed in claim 1, wherein the refractive index of first medium is determined with Fresnel's formulas. 4) A method as claimed in claim 1, wherein the incident light ray is coded. 5) A method as claimed in claim 4, wherein the intensity of the incident light ray is varied according to a periodic signal. 6) A method as claimed in claim 4, wherein the mean intensity of the incident light ray is varied according to a square-wave signal. 7) A method as claimed claim 1, wherein a source light ray is divided so as to form a reference light ray and said incident light ray. 8) A method as claimed in claim 7, wherein said source light ray is divided by reflection and refraction on a second interface. 9) A method as claimed in claim 7, comprising using said first medium whose refractive index is known, measuring the intensity of said reference light ray and the intensity of the reflected light ray, and determining the ratio of the intensity of said reference light ray to the intensity of said incident light ray using a formula which relates the intensity of the incident light ray to the intensity of the reflected light ray at said first interface by taking account of the refractive indices of first medium and of second medium. 10) A method as claimed in claim 7, wherein the intensity of the reference light ray is measured to determine the intensity of the incident light ray. 11) A method as claimed in claim 1, wherein the intensity of the light ray refracted in first medium is measured and absorption of the refracted light ray by first medium is determined. 12) A method as claimed in claim 1, wherein the angle of incidence of the incident light ray in relation to the normal direction to the first interface is smaller than the critical angle. 13) A method as claimed in claim 1, wherein the angle of incidence of the incident light ray in relation to the normal direction to the first interface is below 10°. 14) A method as claimed in claim 1, wherein said first medium is an opaque medium. 15) A method as claimed in claim 1, wherein said first medium is a crude petroleum. 16) A refractometer comprising a light source, a first interface consisting of a first medium of unknown index and of a second medium of known index and at least one detector measuring the intensity of an incident light ray and the intensity of a reflected light ray resulting from the reflection of said incident light ray on said first interface. 17) A refractometer as claimed in claim 16, wherein said first medium is contained in an enclosure comprising a shell and second medium. 18) A refractometer as claimed in claim 16, comprising a means for coding the light ray emitted by said light source and a means for decoding the measurements performed by said detector, said decoding means exchanging information with coding means. 19) A refractometer as claimed in claim 16, comprising an optical element suited to divide a light ray into two light rays so as to form a measuring ray and said incident ray, the intensity of said measuring ray being measured by a detector. 20) A refractometer as claimed in claim 19, wherein optical element consists of a second interface formed by the surface of first medium located outside the enclosure. 21) A refractometer as claimed in claim 19, wherein optical element is a semireflecting blade. 22) A refractometer as claimed in claim 16, comprising a detector measuring the intensity of a light ray refracted in first medium. 23) A refractometer as claimed in claim 16; wherein the angle of incidence of the incident light ray in relation to the normal direction to first interface is smaller than the critical angle. 24) A refractometer as claimed in claim 16, wherein the angle of incidence of the incident light ray in relation to the normal direction to first interface is below 10°. 